The present invention relates to self-calibration of feed cables to an array antenna, and more specifically it relates to calibration of antenna feed cables in a duplex configuration where the same feed cables for a receive direction are also used in the transmit direction.
Antenna arrays are more and more given the attention to give a boost of capacity to cellular networks as opposed to single sector antennas. These array antennas consist of several radiator groups connected together to give a main radiation direction while keeping radiation down in other directions.
However, in order for the array antenna to work properly, coherent signals are necessary in the aperture of the antenna. That is, we need some control of the phase of each signal in each antenna element of the array in order to shift a constructive interference to a desired direction. If this is accomplished, we have a steerable array antenna at our use.
The present technique for implementing array antennas is to use switched beams. In this way, the beam forming can be made once-and-for-all in a passive radio frequency (RF) network that can be connected to the antenna connectors at the top of the mast. In this case no calibration is needed of feed cables or the base station internally.
In other cases where calibration is needed (full steering of antenna beams), the feed cables are carefully measured and calibration equipment is installed internally in the base station. It is then relied on that the phase errors in the cables do not change too much over temperature and time.
It should be observed that it is generally only the transmit direction that needs some calibration. The direction of reception is already self-calibrated, because the signals can be made coherent as they are received by the radio.
However, on transmit, to make antenna signal coherent, it would mean installation of sensors at the top of the antenna mast and some additional control equipment.
The drawback of today""s solution is that calibration of antenna feed cables at transmit frequency (and direction) requires some type of sensors in direct contact with the antenna connectors at the top of the antenna mast. It is not unusual that the height can be of the order 50 meters, so any additional active device at the antenna level is highly disliked by the operator in view of maintenance. On the other hand, if calibration of an antenna array is not implemented, one is forced to use switched beam solutions. This might in turn mean that nulling cannot be performed and that continuous beam steering is not possible. Gain drop in-between fixed beam directions is also a result of non-calibrated systems.
Therefore there is a definite demand for a self-calibration of array antenna feed cables to facilitate a continuous beam steering with coinciding receive and transmit directions in duplex operation configurations of cellular network base stations.
The receive part of a cellular network base station system can be interpreted as self-calibrating and usually does not represent any problem. Instead the main concern is to be directed towards the transmit direction of the base station. The proposed method and system according to the present invention makes it is possible to utilize a common information from the feed cables to be used by both receive frequency and transmit frequency of the base station.
In the receive direction, algorithms tend to optimise the best performance by adding an appropriate phase to antenna branches. The present application teaches how the same thing also can be performed for the corresponding transmit direction by using the phase compensations of the receiving direction also in the transmit direction, but with a proportional correction for the difference in transmit frequency.
A method according to the present invention is set forth by the independent claim 1 and further embodiments are set forth by the dependent claims 2 to 3. Correspondingly a system using the present invention is set forth by the independent claim 4 and further embodiments are set forth by the dependent claims 5 to 6.